The courtyard is lot property but I thought the pebblecrete surface might be common property ?
If the terrace is lot property, you probably only need approval from your strata committee and you would have to show your plans to get their OK. They may be concerned about changing the external appearance of the scheme as well as drainage etc.
If the surface is common property, then you will need special resolution by-laws if you are planning to dig it up but probably not if you are only planning to cover it.
Check your registered plan to see if the terrace is marked as common property.